The three most comprehensive citation searching resources are Scopus, Web of Science,and Google Scholar. While the resources are continually expanding their scope, no tool contains all articles and citations and one resource may perform much better for one researcher than another.
Generally, Web of Science and Scopus offer thorough coverage of the sciences. BGSU subscribes to Web of Science. Google Scholar also has good coverage of the sciences, but its tools are less robust and there is little quality control. It's best to try each of these databases and see which one offers you the best results.
The social sciences and arts & humanities have reasonable journal coverage in Web of Science, but does not cover books extensively. If you are in one of these fields, consider trying JSTOR or EBSCOhost in addition to Google Scholar. You can find instructions for each under the Additional Resources tab.